Of various semiconductor light-emitting devices having a light-emitting layer section composed of compound semiconductors formed therein, those used as light-emitting diode light sources for display or lighting have a metal electrode applying drive voltage to the light-extraction surface side of the light-emitting layer section formed therein. The metal electrode acts as a light interceptor, and is typically formed so as to cover only the center portion of the main surface of the light-emitting layer section, so as to make it possible to extract the light from the surrounding non-electrode-forming area. The metal electrode, however, remains as a light interceptor, and raises another problem in that an extreme reduction in the electrode area inhibits current spreading in the device plane, and undesirably limits the extractable light energy. It has, therefore, been proposed that the entire surface of the light-emitting layer section is covered with an ITO (indium tin oxide) transparent electrode layer having a high electro-conductivity, aiming at achieving both of improvement in the light extraction efficiency through the transparent electrode layer and improvement in the current spreading effect, as typically disclosed in Japanese Laid-Open Patent Publication “Tokkaihei” Nos. 1-225178 and 6-188455.
Both publications pointed out a problem in that formation of the ITO transparent electrode layer directly on the light-emitting layer section results in an excessively high contact resistance, and disables operation at an appropriate operation voltage. Japanese Laid-Open Patent Publication “Tokkaihei” No. 1-225178 discloses a method of epitaxially growing a contact layer composed of InxGa1-xAs (also referred to as InGaAs, hereinafter) directly on the light-emitting layer section by the MOVPE (metal-organic vapor phase epitaxy) process, and further forming thereon the ITO transparent electrode layer. On the other hand, Japanese Laid-Open Patent Publication “Tokkaihei” No. 6-188455 discloses a method of epitaxially growing a GaAs layer on the light-emitting layer section by the MOVPE process, forming thereon the ITO transparent electrode layer, and then annealing it.
This invention is to provide a method of fabricating a light-emitting device, in which the ITO transparent electrode layers as the light-emission drive electrodes are bonded as being underlain by the contact layers, to thereby reduce contact resistance of these electrodes, and to thereby make the contact layers less susceptible to difference in the lattice constants with those of the light-emitting layer section during the formation thereof, and also to provide a light-emitting device further improved in the performance through structural improvement in the contact layer.
The InGaAs contact layer formed by the MOVPE process adopted in the aforementioned prior arts differs in the lattice constants by as large as 4% or around in maximum on the alloy composition basis, from those of a compound semiconductor composing the light-emitting layer section, such as GaAs, or AlGaAs or AlGaInP grown epitaxially on a GaAs substrate. This raises problems below.
(1) This is highly causative of quality degradation such as lowering in the light emission efficiency, due to lattice mismatching between the InGaAs contact layer and the light-emitting layer section.
(2) It is difficult for the method disclosed in Japanese Laid-Open Patent Publication “Tokkaihei” No. 1-225178, in which the InGaAs contact layer is directly grown by epitaxy, to uniformly grow the contact layer, due to the above-described difference in the lattice constants with those of the light-emitting layer section, and this tends to result in island-patterned film formation, to thereby fail in securing a satisfactory contact with the ITO transparent electrode layer. On the other hand, an embodiment disclosed in Japanese Laid-Open Patent Publication “Tokkaihei” No. 6-188455, in which the ITO transparent electrode layer is formed and then annealed, adopts an annealing temperature of as high as 800° C., and an annealing time of as long as 5 minutes.